原式=1/(1*4)+1/(4*7)+1/(7*10)+1/(10*13)=(1/3)*(1-1/4)+(1/3)*(1/4-1/7)+(1/3)*(1/7-1/10)+(1/3)*(1/10-1/13)=(1/3)*(1-1/4+1/4-1/7+1/7-1/10+1/10-1/13)=(1/3)*(1-1/13)=4/13.